The ability to recycle cemented carbide scrap is of great interest since the raw material used when making cemented carbide is expensive. Using recycled cemented carbide will reduce the energy consumption and the environmental impact significantly as compared to using virgin raw materials. However, some cemented carbide products cannot be made from recycled cemented carbide without sacrificing product quality compared to when virgin raw materials is used.
Recycling of cemented carbides is usually done by metallurgical or chemical means, e.g. by the zinc recovering process, electrolytic recovery and, extraction or oxidation. In the Zn recovery process, sometimes also called PRZ (process recycled zinc), the cemented carbide scrap is immersed into molten zinc in an electrical furnace. The zinc is then distilled at a certain temperature and is thus removed from the furnace. The remaining powder comprises the WC and the binder metal, usually cobalt, separated from each other. This is described in e.g. U.S. Pat. No. 3,595,484.
Some cemented carbide grades comprises a gamma phase comprising cubic carbides, e.g. NbC, TiC and TaC. The grain size of this gamma phase has an impact on the final properties of the material. Hence, it is important to control the grain growth during sintering for the gamma phase in order to avoid abnormal grain growth and gamma phase clustering. One way to control the grain growth is described in EP 1 526 189 A1 where the cubic carbides are pre-alloyed with WC. The composition of the pre-alloyed powder should be as close as possible to the equilibrium composition of the gamma phase at the sintering temperature. This results in a fine grained gamma phase. The drawback of this method is that only virgin raw materials can be used in order to obtain the desired microstructure. When recycling cemented carbides containing gamma phase, the resulting powders can have a composition other than that of the composition at equilibrium at the sintering temperature.
Making cemented carbide having two fractions of WC where the two fractions have different grain sizes, i.e. a bimodal WC grain size distribution is known in the art.
EP 0 665 308 A1 describes a cemented carbide having a bimodal WC grain size distribution. The obtained cemented carbide has an increased resistance towards plastic deformation in comparison to a cemented carbide having a normal grain size distribution.
EP 0 913 489 B1 describes a cemented carbide comprising gamma phase having a bimodal WC grain distribution where some of the WC is added as powder recovered by the Zn recovery process. The cubic carbides forming the gamma phase are all added as virgin materials.
One object of the present invention is to obtain a method of making cemented carbide having a fine grained gamma phase using recycled cemented carbide comprising cubic carbides as a raw material.
Another object of the present invention is to make a cemented carbide having a fine grained gamma phase having the same or improved properties compared with a cemented carbide having a fine grained gamma phase made from virgin raw material.
It has been discovered that by pre-milling the PRZ-powder to a fine grain size prior to mixing it with a coarser powder fraction, a cemented carbide having the same or improved properties as compared to cemented carbides made from virgin materials can be achieved.